1. Field of the Invention
The present invention relates to a surface acoustic wave convolver in which surface acoustic wave transducers respectively provided with interdigital electrodes having different electrode widths and periods (pitches) in a direction of propagation of a surface acoustic wave are combined.
2. Description of the Prior Art
A conventional surface acoustic wave transducer having interdigital electrode each comprising positive and negative electrodes formed on a piezoelectric substrate (including a piezoelectric thin film substrate) or electrostrictive substrate generally has an electrode layout structure in which the positive and negative electrodes are arranged at equal periods. FIG. 9(a) is a plan view showing a surface acoustic wave convolver using conventional surface acoustic wave transducers, and FIG. 9(b) is a sectional view thereof along the line X-Y in FIG. 9(a). Referring to FIGS. 9(a) and 9(b), reference numeral 51 denotes a first surface acoustic wave transducer for converting an electrical signal into a surface acoustic wave; 52, a second surface acoustic wave transducer for converting an electrical signal similar to the one described above into a surface acoustic wave; and 53, an output electrode for detecting the surface acoustic waves generated and propagating from the surface acoustic wave transducers 51 and 52 to extract a convolution output as an electrical signal. Each interdigital electrode of the surface acoustic wave transducer 51 or 52 has an electrode layout structure in which the positive and negative electrodes are arranged at equal periods. That is, if an electrode width in the interdigital electrode is defined as m and a period is defined as p, the period p is constant. In addition, ratios m/p are constants (mostly 0.5) regardless of positions on the interdigital electrode.
In the surface acoustic wave transducer having this electrode layout structure, surface acoustic waves generated by this transducer propagate to the right and left with substantially the same amplitude. Thus, this surface acoustic wave transducer has similar insertion loss characteristics in the tow directions, i.e., so-called bi-directional characteristics.
Conventional techniques each for obtaining a uni-directional surface acoustic wave transducer having a low insertion loss in only one direction by using an electrode layout structure in which the positive and negative electrodes are arranged at equal periods are exemplified by a method using a 120.degree. phase shifter, a method using a 90.degree. phase shifter, and a method of obtaining an internal reflection type uni-directional transducer in which reflection electrodes are asymmetrically arranged between positive and negative electrodes at equal periods.
The surface acoustic wave transducer having the electrode layout structure in which the positive and negative electrodes are arranged at equal periods has bi-directional characteristics but cannot provide characteristics having a low insertion loss in only one direction. Even if such surface acoustic wave transducers are used to constitute a surface acoustic wave convolver, the convolver does not have high convolution efficiency. In the method of obtaining the uni-directional surface acoustic wave transducer, characteristics having a low insertion loss in only one direction can be obtained. However, since the surface acoustic wave transducers each having the electrode layout structure in which the positive and negative electrodes are arranged at equal periods are used, wide-range characteristics cannot be obtained. Therefore, a surface acoustic wave convolver employing this surface acoustic wave transducer cannot provide wide-range characteristics, either.